This invention relates generally to the production of tetrabasic lead sulfate and more specifically to the production of tetrabasic lead sulfate from solid state reactions for the preparation of active plates to be used in lead-acid batteries.
The preparation of active electrodes or plates for lead-acid batteries involves three steps: a) paste application to a supporting grid, b) drying and curing of the pasted grid, and c) electrochemical formation of the cured grid to produce the active materials used in the charged lead-acid battery. Battery formation is usually carried out by passing current through the electrode, which results in a positive plate having pellets of lead dioxide, and a negative plate having pellets of metallic lead. Tetrabasic lead sulfate, 4PbO.PbSO4, is a very useful starting material for the preparation of positive electrodes for lead-acid batteries. According to U.S. Pat. Nos. 3,765,943 and 3,899,349, battery plates or electrodes fabricated with this sulfate and cured in the presence of a controlled amount of carbon dioxide gas, are associated with an increased product yield, greater life, greater reproducibility in the chemical and physical properties of the product, and crystal morphology more suitable for battery operation. Although both positive and negative electrodes may be formed with tetrabasic lead sulfate, this material is preferably used for positive plates. See also Pavlov, D. and Papazov, G., Journal of Applied Electrochemistry 6, 1976, pp. 339-345, as well as Pavlov, D. and Kapkov, N. J., Electrochemical Society 137(1), 1990, pp. 16-28.
In the manufacturing process of lead-acid batteries, tetrabasic lead sulfate is usually produced by mixing the required amounts of leady oxide (α and β-PbO with free lead), sulfuric acid and water. Although this procedure seems to be very straightforward, several important variations of it have been reported, mainly in the patent literature.
For instance, U.S. Pat. No. 3,765,943 discloses a process for fabrication of positive electrodes for lead-acid batteries wherein the electrodes arc fabricated from grids filled with paste and including the steps of reacting orthorhombic lead oxide in an aqueous suspension with sulfuric acid at 80-100° C. to produce tetrabasic lead sulfate. The lead-containing material contains at least 90 wt % of lead oxide, the aqueous suspension in which the reaction takes place is preacidified to a pH range of 1-3.5 prior to the addition of lead oxide and the tetrabasic lead sulfate is separated from the aqueous suspension within one hour after complete addition of the sulfuric acid.
U.S. Pat. No. 4,324,768 discloses a process for the preparation of water insoluble lead compounds having a composition represented by the general formula nPbO.PbX2, wherein X stands for an inorganic acid radical, x indicates the valency of the radical X and n is a number of from 0 to 5, which comprises reacting lead monoxide according to the wet method with an inorganic acid or organic acid in the presence of hydroxylamine under such conditions that the initial pH value of the reaction system is not higher than 7. The lead compound represented by the general formula may be an ortho-salt such as lead chromate or a basic salt such as tetrabasic lead sulfate (4PbO.PbSO4). Further, it may be a salt containing at least two acid radicals, such as lead silicosulfate.
U.S. Pat. No. 5,096,611 discloses a process for making a battery paste comprising an initial step of forming a paste mixture by reacting a lead oxide having a low free lead content with sulfuric acid in an aqueous slurry, followed by dewatering of the partially reacted slurry to obtain the paste material. In one described embodiment, a slurry containing one or more basic lead sulfates suitable for use as the active material in lead-acid battery electrodes is formed in a continuously stirred tank reactor. The slurry is withdrawn from the reactor and fed to a belt press, which reduces the moisture content of the slurry to the desired level. Battery pastes produced according to that invention may be fed continuously to a paster for mass production of positive and negative battery plates from grids. This process can utilize a variety of lead oxides including Barton, leady and nonleady oxides of any crystal structure, and can produce a paste containing either tribasic lead sulfate, tetrabasic lead sulfate, or a mixture thereof at a specified ratio. The amount of tetrabasic lead sulfate in the battery paste is determined during paste formation, and no subsequent adjustment is needed. Free lead can be removed or oxidized and the curing step is eliminated from the lead-acid battery manufacturing process. U.S. Pat. No. 5,290,359 discloses an apparatus for making a battery paste following the process disclosed in U.S. Pat. No. 5,096,611.
U.S. Pat. No. 5,252,105 discloses a continuous process in which an electrode for a lead-acid battery is prepared without the conventional curing step. The general procedure for preparing electrodes includes preparing a paste mixture comprising an active material precursor, sulfate-containing acids, and an inhibitor. The active material precursor includes lead oxides having at least 10% by weight lead oxide in the form of Pb3O4 (red lead), and a BET surface area of at least about 0.8 m2/g; desirably about 1.00 to 1.50 m2/grain and preferably about 1.0 to 1.25 m2/g. The inhibitor (preferably a simple sugar or a related compound having at least one hydroxyl group per carbon atom) prevents formation of tribasic lead sulfate and tetrabasic lead sulfate from the precursor material and sulfate-containing acids, except at elevated temperatures (tetrabasic lead sulfate forms with further processing at temperatures typically in excess of 80° C). The paste is applied to electrode grids and reacted at elevated temperatures for between about 5 and about 30 minutes, to form the active material of the electrode for both positive and negative electrodes. Plates are then assembled into batteries and charged. Negative electrodes differ from the positive, mainly in the additives used.
U.S. Pat. No. 5,273,554 discloses a process for fabricating a battery in which tetrabasic lead sulfate is produced by reacting lead oxide with a substantial excess of a sulfate reactant at a pH in the range of 9.3 to 12. The resulting materials provide needle-like structures with a width generally in the range of 3 to 1 μm. The relative narrow needles, when employed on the positive electrode of a lead acid battery, improve the efficiency of formation, provide good adhesive to the positive plate, extend battery life, as well as, yield excellent capacity per gram of active material. In one embodiment, a precursor is prepared by reacting lead oxide with sulfuric acid in the presence of an excess of sulfate to form a paste and applying the paste to a grid with subsequent curing. Control of the temperature below 60° C. in the reaction medium and of the sulfate excess yields, after curing at a temperature above 70° C. and 100% humidity, a positive plate having an extremely uniform prismatic size, tetrabasic lead sulfate and a uniform pore distribution. Most significantly, the width of these crystals is extremely narrow—having an average dimension in the range of 1 to 2 μm—allowing rapid conversion to lead oxide from the precursor and further providing enhanced adhesion and current capacity attributes. Thus, the lead oxide obtained after conversion has prismatic crystals with an average crystal width less than 2.5 μm. In a second embodiment, tetrabasic lead sulfate of relative small prismatic size is achieved by just reacting lead oxide with sulfuric acid in a stirred aqueous solution containing excess sulfate at temperatures above 60° C. to immediately form tetrabasic lead sulfate. The tetrabasic lead sulfate is mixed with water to form a paste. Positive plates made by applying this paste to a lead grid and converted to lead oxide, also have higher current capacity attributes. U.S. Pat No. 5,660,600 refers to the batteries fabricated according to the process disclosed in U.S. Pat. No. 5,273,554, as well as to the materials useful therein.
U.S. Pat. No. 6,454,977 discloses a continuous paste making process for lead-acid batteries that includes the steps of mixing water with a lead oxide, reacting an acid (usually sulfuric acid) with the lead oxide in a mixture to produce lead oxide-lead sulfate compounds, and forming a paste comprising interlocking basic lead sulfate complex crystals from the lead lead-oxide sulfate compounds, where the mixing, reacting, and crystal forming steps occur in an extrusion or in a high-shear continuous processing apparatus. The method also includes the step of extruding the paste from the extrusion apparatus into a grid mesh where the paste is dried to form a battery plate of the lead-acid battery.
Evidently, tetrabasic lead-sulfate is an essential raw material for the lead-acid battery industry. Surprisingly, only a few synthesis methods, other than the ones mentioned above, have been reported in the scientific literature for the production of tetrabasic lead sulfate.
For instance, Burbank J., Journal of The Electrochemical Society 113(1), 1966, pp. 10-14, describes a method for the preparation of monobasic, tribasic and tetrabasic lead sulfates by reacting PbO and dilute sulfuric acid. The crystalline phases were identified by x-ray diffraction, and electron microscope examination showed all three to consist of prismatic needles. The three basic sulfates were pressed into pellets and oxidized anodically in a frame of pure lead and in 1.050 specific gravity sulfuric acid X-ray diffraction showed that each transformed to alpha lead dioxide. PbO2 formed from pellets made from tetrabasic lead sulfate were mechanically strong formed by the interlocking of large crystals.
Vilhunen, Journal of Power Sources 39, 1992, pp. 59, evaluated the differences arising on the positive active material with high tetrabasic lead sulfate content due to preparation technique or starting material. Wet chemical analysis, X-ray diffraction, transmission and scanning electron microscopy methods were used to characterize the materials. The tetrabasic lead sulfate was made from leady oxide and tetragonal PbO by solution or paste methods. The paste yielded smaller crystals which were also more irregular than the large single crystal-like particles resulting from the solution process. These results provide choices for achieving the crystal structure in conditional plates most conducive to long cycle life.
Lastly, Grugeondewaele et al., Journal of Power Sources 72(2), 1998, pp. 126-131, prepared tribasic lead sulfate and tetrabasic lead sulfate by reactive grinding. The effects of various experimental parameters (stoichiometry, hygrometry of the starting compounds, duration of mechanical treatment) upon the nature and morphological features of the resulting phase were investigated. With water in excess, only tribasic lead sulfate was produced while dry reagents led to tetrabasic lead sulfate. In both case, samples with a small particle size and high reactivity were obtained. In order to evaluate the influence of the morphology upon the electrochemical performances of such grinding produced samples, the capacity was measured and compared with that of traditional tribasic lead sulfate and tetrabasic lead-sulfate samples.
In brief, the available methods for the production of tetrabasic lead sulfate are complex, costly or yield heterogeneous crystal sizes.